Sewing by a sewing machine is performed while a cloth is held by a presser foot. The presser foot has a primary function of suppressing, when a needle is pulled out from a cloth, an uplifting of the cloth associated with the pulled-out needle. In addition, the presser foot has a secondary function of holding the cloth together with a feed dog, and smoothly feeding the cloth. In order to fully accomplish such functions, it is necessary to appropriately control depress force applied to the cloth from the presser foot.
A sewing machine intertwines a needle thread with a bobbin thread, thereby forming a seam. When the seam is too tight, a cloth with high stretch properties, such as a jersey cloth or a knit cloth, causes a material puckering, and when the seam is too loose, the threads are likely to come apart from each other. Hence, it is necessary to control the tension of thread (stitch balancing thread tension) based on the thickness of a cloth and the stretch properties thereof.
In order to forma stitchwork on the cloth, the cloth is held on not a feed dog but a stitchwork frame, and is translated in the vertical and horizontal directions by a frame driving mechanism. Hence, when a sewing machine is utilized for a stitchwork formation, a presser foot is lifted up from the surface of the cloth by a predetermined distance before the stitchwork formation starts, and then the stitchwork formation is started (see, for example, JP2006-20757 A). In order to cause the presser foot to properly function during the stitchwork formation, it is necessary to properly control the positional relationship between the presser foot and the cloth based on the thickness of the cloth.
In addition, when a stitchwork formation is performed on a cloth with high stretch properties like a quilt, the actual height of the presser foot is likely to be lower than the set distance relative to the cloth, causing an improper sewing. Hence, as for a cloth with high stretch properties, it is necessary to properly control the positional relationship between the presser foot and the cloth based on the thickness of the cloth and the stretch amount thereof.
As explained above, according to conventional technologies, like the disclosure of JP2006-20757 A, the thickness of a cloth is detected, and a sewing condition is set in accordance with the thickness of the cloth. As for the thickness of the cloth, the presser foot is moved down toward the cloth, and when the presser foot stops due to repulsive force from the cloth, the height of the presser foot at this time point is determined as the thickness of the cloth.
However, all cloths naturally have stretch properties. When the stop height of the presser foot is set as the thickness of a cloth, the thickness of the cloth depressed by the presser foot is to be detected, and thus it is difficult to detect the thickness of the cloth when no load is applied. In addition, the stretch properties of a cloth vary depending on a cloth type. That is, the depress amount of a cloth by the presser foot varies depending on the cloth type, and it is difficult to set the thickness of the cloth when no load is applied based on the thickness of the depressed cloth.
Hence, according to conventional cloth-thickness detecting schemes, it is difficult to set a sewing condition in accordance with the thickness of a cloth when no load is applied, which does not contribute to an improvement of sewing quality and stitchwork quality. In fact, when an actual difference between the thickness of the cloth when no load is applied and the thickness of the cloth based on the conventional cloth-thickness detection is large, a sewing condition set up based on the conventional schemes sometimes results in a deterioration of sewing quality and stitchwork quality.
The present invention has been proposed in order to address the above technical problems of conventional technologies, and it is an objective of the present invention to provide a sewing machine which is capable of detecting a thickness of a cloth and stretch properties thereof when no load is applied, and which is also capable of improving a sewing quality and a stitchwork quality through a setting of a sewing condition based on the detected thickness and stretch properties.